In conventional computerized tomography for both medical and industrial applications, an x-ray fan beam and a linear array detector are employed to achieve two-dimensional axial imaging. The quality of these two-dimensional (2D) images is high, although only a single slice of an object can be imaged at a time. To acquire a three-dimensional (3D) data set, a series of 2D images are sequentially obtained in what is known as the “stack of slices” technique. One drawback to this method is that acquiring the 3D data set one slice at a time is an inherently slow process. There are other problems with this conventional tomographic technique, such as motion artifacts arising from the fact that the slices cannot be imaged simultaneously, and excessive exposure to x-ray radiation due to overlap of the x-ray projection areas.
Another technique for 3D computerized tomography is cone-beam x-ray imaging. In a system employing cone-beam geometry, an x-ray source projects a cone-shaped beam of x-ray radiation through the target object and onto a 2D area detector area. The target object is scanned, preferably over a 360-degree range, either by moving the x-ray source and detector in a scanning circle around the stationary object, or by rotating the object while the source and detector remain stationary. In either case, it is the relative movement between the source and object which accomplishes the scanning. Compared to the 2D “stack of slices” approach for 3D imaging, the cone-beam geometry is able to achieve 3D images in a much shorter time, while minimizing exposure to radiation. One example of a cone beam x-ray system for acquiring 3D volumetric image data using a flat panel image receptor is discussed in U.S. Pat. No. 6,041,097 to Roos, et al.
A significant limitation of existing cone-beam reconstruction techniques occurs, however, when the object being imaged is larger than the field-of-view of the detector, which is a quite common situation in both industrial and medical imaging applications. In this situation, some measured projections contain information from both the field of view of interest and from other regions of the object outside the field of view. The resulting image of the field of view of interest is therefore corrupted by data resulting from overlying material.
Several approaches have been proposed for imaging objects larger than the field-of-view of the imaging system. U.S. Pat. No. 5,032,990 to Eberhard et al., for example, discusses a technique for 2D imaging of an object which is so wide that a linear array detector is not wide enough to span the object or part which is to be viewed. The method involves successively scanning the object and acquiring partial data sets at a plurality of relative positions of the object, x-ray source, and detector array. U.S. Pat. No. 5,187,659 to Eberhard et al. discusses a technique for avoiding corrupted data when performing 3D CT on an object larger than the field of view. This technique involves scanning the object with multiple scanning trajectories, using one or more x-ray sources and detectors which rotate in different trajectories relative to the target object. Yet another technique is discussed in U.S. Pat. No. 5,319,693 to Ebarhard et al. This patent discusses simulating a relatively large area detector using a relatively small area detector by either moving the actual area detector relative to the source, or moving the object relative to the area detector. All of these techniques are characterized by complex relative movements between one or more x-ray sources, detectors, and the object being imaged. Furthermore, in each of these techniques, the target object is exposed to excessive x-ray radiation from regions of overlapping projections.
To date, there does not exist a radiation system for imaging large field-of-view objects in a simple and straightforward manner while minimizing the target object's exposure to radiation.